Spraying Fort Collins' southeast corner for adult mosquitoes dramatically reduced the number of West Nile virus-infected mosquitoes in the area, data from the Fort Collins Department of Health and Environment shows.

When county health Director Adrienne LeBailly compares data from the area sprayed and the surrounding part of the city's southeast quadrant that wasn't sprayed, the success of the spraying decision is clear, she said.

However, LeBailly has been criticized for overstepping the city of Fort Collins' protocol when it comes to spraying for adult mosquitoes, and for spraying even though trap data shows the threat of West Nile in southeast Fort Collins dropped shortly before spraying began.

"There is only one trap that is positive," LeBailly said of post-spray results taken from mosquito traps in the sprayed area of southeast Fort Collins this weekend. "If you go back to week 32 (data collected in early August), it's a big drop."

Larimer County had Colorado Mosquito Control spray a section of Fort Collins southeast of Harmony Road the intersection of Harmony Road and Lemay Avenue, and an abutting portion of unincorporated Larimer County, on Aug. 15 and 18.

LeBailly said the area sprayed saw the risk of human infection — called the vector index — drop to 0.14 in the area sprayed, whereas the portion of southeast Fort Collins that went unsprayed carried a vector index of 0.55 in the most recent trap results. The vector index in all of southeast Fort Collins measured at 0.35.

But LeBailly's critics — namely the advocacy group No Spray Fort Collins — say the spraying was unnecessary, as the vector index had dropped to an even lower level days before spraying started.

"The most recent trap data ... indicate that the vector index in southeast Fort Collins had spontaneously declined from 0.634 to 0.270 before spraying began last Friday," said No Spray Fort Collins' main voice, Dana Kunze, in an email to the Coloradoan. "The data is evidence that both the city and county constituents have now been exposed to both the financial impact and health risks of spraying without any justification whatsoever."

LeBailly argues that cool weather earlier in the week might have artificially reduced the number of West Nile positive traps, as mosquito activity ebbs and flows along with high temperatures.

"I wasn't convinced that was necessarily a reliable number," she said. "Very clearly, it's gone down significantly, and it had been steadily going up."

A recent stretch of cooler weather has knocked down the human risk of disease across the city, as the vector index is at 0.177 or less all areas outside the city's southeast corner.

Despite "a lot of unhappiness among certain city officials," LeBailly said she has received more positive feedback from area residents thanking her for spraying. A Colorado Mosquito Control employee who sprayed the neighborhoods told her that, for the first time ever, residents were giving him a thumbs-up as he drove through town.

Not everybody shares that sentiment, though, and both county and city officials have pledged to discuss the entities' divergent spraying policies in the future.

By the numbers

County health officials broke down the southeast quadrant of the city into areas it had sprayed for West Nile virus on Aug. 15 and 18, and the remaining unsprayed area. Trap data was most recently collected Aug. 18-21.

Sprayed area: One positive West Nile trap of eight total traps. Average vector index of 0.14.

Unsprayed area: Six positive West Nile traps of 10 total traps. Average vector index of 0.55.